Update DziTileSource class constructor

[geoff-harper]

Fixed DZI tile source constructor. There's a bit of dissonance between the docs and the source code so I tried to split the difference with a sensible solution. I've confirmed that my implementation will load the DZI

[alvaromartmart]

So sorry for the delay @geoff-harper, and thanks for the contribution.
I actually managed to contribute the type definitions to DefinitelyTyped, maybe it's better if moving forward we keep mantaining the types there.

I'll also update this repo's readme so future contributors go directly there